The "Pittsburgh Pneumonia Agent" (PPA) is a newly-recognized pathogen which causes fatal pneumonia in immunocompromized patients. This gram-negative, weakly acid-fast bacterium resembles Legionella pneumophila in disease-producing properties but has been shown to differ from L. pneumophila by microbiological and seriological criteria. At present PPA can only be cultivated in embryonated eggs or guinea pigs. Attempts will be made to improve intra-vitum diagnosis of disease by improving established cultural and immunofluorescent techniques. The antibiotic susceptibility of PPA will be determined in-vitro and in-ovo and compared with the response of infected model animals and patients. The natural history and epidemiology of PPA infection in special and normal populations will be studied. Retrospective autopsy studies will estimate the prevalence of disease. We will undertake prospective cultural and serological studies of pneumonia in renal transplant recipients and pneumonia patients. The pathogenesis of PPA disease will be studied in a guinea pig model. The effects of immunosuppression on immunity will be examined. The DNA composition of the bacterium will be determined to discover any relationship between PPA and other known bacteria. This will be determined by DNA-DNA hybridization tests.